## Maleficent dark matter: Part II

In Part I of the series we saw how dark matter could cause mass extinction by inducing biosphere-wide cancer, stirring up volcanoes, or launching comets from the Oort cloud. In this second and final part, we explore its other options for maleficence.

World-devouring dark matter

The dark matter wind that we encountered in Part I has yet another trick to bring the show on this watery orb to an abrupt stop. As J. F. Acevedo,  J. Bramante, A. Goodman, J. Kopp, and T. Opferkuch put it in their abstract, “Dark matter can be captured by celestial objects and accumulate at their centers, forming a core of dark matter that can collapse to a small black hole, provided that the annihilation rate is small or zero. If the nascent black hole is big enough, it will grow to consume the star or planet.” Before you go looking for the user guide to an Einstein-Rosen bridge, we draw your attention to their main text: “As, evidently, neither the Sun nor the Earth has suffered this fate yet, we will be able to set limits on dark matter properties.” For once we are more excited about limits than discovery prospects.

R-rated dark matter

Enough about destructions of life en masse. Let us turn to selective executions.

Macro dark matter” is the idea that dark matter comprises not of elementary particles but composite objects that weigh anywhere between micrograms and tonnes, and scatter on nuclei with macroscopic geometric cross sections. As per J. J. Sidhu, R. J. Scherrer and G. Starkman, since the dark wind blows at around 300 km/s, a dark macro encountering a human body would produce something akin to gunshot or a meteor strike, only more gruesome. Using 10 years of data on the well-monitored human population in the US, Canada and Western Europe, and assuming that it takes at least 100 J of energy deposition to cause significant bodily damage, they derive limits on dark matter cross sections and masses shown in the adjoining figure.

We’re afraid there’s nothing much you can do about a macro with your name on it.

Inciteful dark matter

Dark matter could sometimes kill despite no interactions with the Standard Model beyond gravity. [Movie spoilers ahead.] In the film Dark Matter, a cosmology graduate student is discouraged from pursuing research on the titular topic by his advisor, who in the end rejects his dissertation. His graduation and Nobel Prize dreams thwarted, and confidante Meryl Streep’s constant empathy forgotten, the student ends up putting a bullet in the advisor and himself (yes, in that order). Senior MOND advocates, take note.

Vital dark matter

Lest we suspect by now that dark matter has a hotline to the Grim Reaper’s office, D. Hooper and J. H. Steffen clarify that it could in fact breathe life into desolate pebbles in the void. Without dark matter, rocky planets on remote orbits, or rogue planets ejected from their star system, are expected to be cold and inhospitable. But in galactic regions where dark matter populations are high, it could capture in such planets, self-annihilate, and warm them from the inside to temperatures that liquefy water, paving the way for life to “emerge, evolve, and survive“. The fires of this mechanism would blaze on long after main sequence stars cease to shine!

And perhaps one day these creatures may use the very DNA they got from dark matter to detect it.

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Bibliography.

[6] Dark Matter, Destroyer of Worlds: Neutrino, Thermal, and Existential Signatures from Black Holes in the Sun and Earth, J. F. Acevedo,  J. Bramante, A. Goodman, J. Kopp, and T. Opferkuch, arXiv: 2012.09176 [hep-ph]

[7] Death and serious injury from dark matter, J. J. Sidhu, R. J. Scherrer and G. Starkman, Phys. Lett. B 803 (2020) 135300

[8] Dark Matter and The Habitability of Planets, D Hooper & J. H. Steffen, JCAP 07 (2012) 046

[9] New Dark Matter Detectors using DNA or RNA for Nanometer Tracking, A. Drukier, K. Freese, A. Lopez, D. Spergel, C. Cantor, G. Church & T. Sano, arXiv: 1206.6809 [astro-ph.IM]

Article Title: “FASER: ForwArd Search ExpeRiment at the LHC”

Authors: The FASER Collaboration

Reference: https://arxiv.org/abs/1901.04468

When the LHC starts up again for its 3rd run of data taking, there will be a new experiment on the racetrack. FASER, the ForwArd Search ExpeRiment at the LHC is an innovative new experiment that just like its acronym, will stretch LHC collisions to get the most out of them we can.

While the current LHC detectors are great, the have a (literal) hole. General purpose detectors (like ATLAS and CMS) are essentially giant cylinders with the incoming particle beams passing through the central axis of the cylinder before colliding. Because they have to leave room for the incoming beam of particles, they can’t detect anything too close to the beam axis. This typically isn’t a problem, because when a heavy new particle, like Higgs boson, is produced, its decay products fly off in all directions, so it is very unlikely that all of the particles produced would end up moving along the beam axis. However if you are looking for very light particles, they will often be produced in ‘imbalanced’ collisions, where one of the protons contributes a lot more energy than the other one, and the resulting particles therefore mostly carry on in the direction of the proton, along the beam axis. Because these general purpose detectors have to have a gap in them for the beams to enter they have no hope of detecting such collisions.

That’s where FASER comes in.

FASER is specifically looking for new light “long-lived” particles (LLP’s) that could be produced in LHC collisions and then carry on in the direction of the beam. Long-lived means that once produced they can travel for a while before decaying back into Standard Model particles. Many popular models of dark matter have particles that could fit this bill, including axion-like particles, dark photons, and heavy neutral leptons.  To search for these particles FASER will be placed approximately 500 meters down the line from the ATLAS interaction point, in a former service tunnel. They will be looking for the signatures of LLP’s that made were produced in collisions at the ATLAS interaction point, traveled through the ground and eventually decayed in volume of their detector.

Any particles reaching FASER will travel through hundreds of meters of rock and concrete, filtering out a large amount of the Standard Model particles produced in the LHC collisions. But the LLP’s FASER is looking for interact very feebly with the Standard Model so they should sail right through. FASER also has dedicated detector elements to veto any remaining muons that might make it through the ground, allowing FASER  be able to almost entirely eliminate any backgrounds that would mimic an LLP signal. This low background and their unique design will allow them to break new ground in the search for LLP’s in the coming LHC run.

In addition to their program searching for new particles, FASER will also feature a neutrino detector. This will allow them to detect the copious and highly energetic neutrinos produced in LHC collisions which actually haven’t been studied yet. In fact, this will be the first direct detection of neutrinos produced in a particle collider, and will enable them to test neutrino properties at energies much higher than any previous human-made source.

FASER is a great example of physicists thinking up clever ways to get more out of our beloved LHC collisions. Currently being installed, it will be one of the most exciting new developments of the LHC Run III, so look out for their first results in a few years!

The FASER Collaboration’s Detector Design Page

Press Release for CERN’s Approval of FASER

Announcement and Description of FASER’s Neutrino program

## Maleficent dark matter: Part I

We might not have gotten here without dark matter. It was the gravitational pull of dark matter, which makes up most of the mass of galactic structures, that kept heavy elements — the raw material of Earth-like rocky planets — from flying away after the first round of supernovae at the advent of the stelliferous era. Without this invisible pull, all structures would have been much smaller than seen today, and stars much more rare.

Thus with knowledge of dark matter comes existential gratitude. But the microscopic identity of dark matter is one of the biggest scientific enigmas of our times, and what we don’t know could yet kill us. This two-part series is about the dangerous end of our ignorance, reviewing some inconvenient prospects sketched out in the dark matter literature. Reader discretion is advised.

[Note: The scenarios outlined here are based on theoretical speculations of dark matter’s identity. Such as they are, these are unlikely to occur, and even if they do, extremely unlikely within the lifetime of our species, let alone that of an individual. In other words, nobody’s sleep or actuarial tables need be disturbed.]

Carcinogenic dark matter

Maurice Goldhaber quipped that “you could feel it in your bones” that protons are cosmologically long-lived, as otherwise our bodies would have self-administered a lethal dose of ionizing radiation. (This observation sets a lower limit on the proton lifetime at a comforting $10^7$ times the age of the universe.) Could we laugh similarly about dark matter? The Earth is probably amid a wind of particle dark matter, a wind that could trigger fatal ionization in our cells if encountered too frequently. The good news is that if dark matter is made of weakly interacting massive particles (WIMPs), K. Freese and C. Savage report safety: “Though WIMP interactions are a source of radiation in the body, the annual exposure is negligible compared to that from other natural sources (including radon and cosmic rays), and the WIMP collisions are harmless to humans.

The bad news is that the above statement assumes dark matter is distributed smoothly in the Galactic halo. There are interesting cosmologies in which dark matter collects in high-density “clumps” (a.k.a. “subhalos”, “mini-halos”,  or “mini-clusters”). According to J. I. Collar, the Earth encountering these clumps every 30–100 million years could explain why mass extinctions of life occur periodically on that timescale. During transits through the clumps, dark matter particles could undergo high rates of elastic collisions with nuclei in life forms, injecting 100–200 keV of energy per micrometer of transit, just right to “induce a non-negligible amount of radiation damage to all living tissue“. We are in no hurry for the next dark clump.

Eruptive dark matter

If your dark matter clump doesn’t wipe out life efficiently via cancer,  A. Abbas and S. Abbas recommend waiting another five million years. It takes that long for the clump dark matter to gravitationally capture in Earth, settle in its core, self-annihilate, and heat the mantle, setting off planet-wide volcanic fireworks. The resulting chain of events would end, as the authors rattle off enthusiastically, in “the depletion of the ozone layer, global temperature changes, acid rain, and a decrease in surface ocean alkalinity.”

Armageddon dark matter

If cancer and volcanoes are not dark matter’s preferred methods of prompting mass extinction, it could get the job done with old-fashioned meteorite impacts.

It is usually supposed that dark matter occupies a spherical halo that surrounds the visible, star-and-gas-crammed, disk of the Milky Way.  This baryonic pancake was formed when matter, starting out in a spinning sphere, cooled down by radiating photons and shrunk in size along the axis of rotation; due to conservation of angular momentum the radial extent was preserved. No such dissipative process is known to govern dark matter, thus it retains its spherical shape. However, a small component of dark matter might have still cooled by emitting some unseen radiation such as “dark photons“. That would result in a “dark disk” sub-structure co-existing in the Galactic midplane with the visible disk. Every 35 million years the Solar System crosses the Galactic midplane, and when that happens, a dark disk of surface density of 10 $M_\odot$/pc$^2$ could tidally perturb the Oort Cloud and send comets shooting toward the inner planets, causing periodic mass extinctions. So suggest L. Randall and M. Reece, whose arXiv comment “4 figures, no dinosaurs” is as much part of the particle physics lore as Randall’s book that followed the paper, Dark Matter and the Dinosaurs.

We note in passing that SNOLAB, the underground laboratory in Sudbury, ON that houses the dark matter experiments DAMIC, DEAP, and PICO, and future home of NEWS-G, SENSEISuper-CDMS and ARGO, is located in the Creighton Mine — where ore deposits were formed by a two billion year-old comet impact. Perhaps the dark disk nudges us to detect its parent halo.

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In the second part of the series we will look — if we’re still here — at more surprises that dark matter could have planned for us. Stay tuned.

Bibliography.

[1] Dark Matter collisions with the Human Body, K. Freese & D. Savage, Phys.Lett.B 717 (2012) 25-28.

[2] Clumpy cold dark matter and biological extinctions, J. I. Collar, Phys.Lett.B 368 (1996) 266-269.

[3] Volcanogenic dark matter and mass extinctions, S. Abbas & A. Abbas, Astropart.Phys. 8 (1998) 317-320

[4] Dark Matter as a Trigger for Periodic Comet Impacts, L. Randall & M. Reece, Phys.Rev.Lett. 112 (2014) 161301

[5] Dark Matter and the Dinosaurs, L. Randall, Harper Collins: Ecco Press‎ (2015)